1. Field of the Invention
The present invention relates in general to a thin-film-transistor liquid-crystal-display(TFT-LCD). In particular, the present invention relates to a flicker-proof thin-film-transistor liquid-crystal-display.
2. Description of the Related Art
The structure of a conventional TFT-LCD is comprised essentially of LCD cells comprising a pair of electrode substrates filled with liquid crystal molecules. Polarizors are adhered to the sides of the electrode substrates. Signal lines and scanning lines are formed perpendicularly with each other forming a matrix on one of the substrates. The scanning lines are connected to each gate of the TFT controlling the on/off state of the TFT and hence the writing of video signals.
Referring to FIGS. 1A and 1B, a pulse signal at the front of the signal scanning line is shown in FIG. 1A. Because of the parasitic resistors and capacitors on the scanning line, the input pulse signal is subjected to RC (time constant) delay. Therefore at the end of the scanning line, the pulse wave is transformed to that shown in FIG. 1B. A voltage coupled from the gate of the TFT is defined as follows:VCOUPLED=VG×Cgs/(Cgs+CLC+CST)where VG is the voltage applied to the gate, Cgs is the capacitance between the gate and the source, CLC is the capacitance of the liquid crystals, and CST is the capacitance of a storage capacitor.
The voltage applied to the gate of the TFT at the front end of the scanning line is VG1, and the voltage applied to the gate of the TFT at the rear end of the scanning line is VG2. In the conventional art, because VG1 is greater than VG2, the coupled voltage VCOUPLED1 is greater than VCOUPLED2. As a result, the LCD display may flicker.
In order to solve the problem of flickering, Japanese Patent Application Laid-Open No. 11-281957 (Sharp Corporation) reduces the gate voltage. That is, the circuit in FIG. 2 is adopted to provide the VVH in FIG. 3 for the driving circuit of the gate of the TFT and the pulse wave generated is as shown in FIGS. 4A and 4B. Referring to FIG. 3, Stc is a trigger voltage for controlling switches SW1 and SW2 such that the circuit is discharged when SW1 is off and SW2 is on resulting in drop of VGH; and the circuit is connected to the power supply Vdd and recharged when SW1 is on and SW2 is off to allow VGH to climb back. Additionally, FIG. 3 shows curve A representing the voltage signal VGH at a lower temperature and curve B representing the voltage signal VVH at a higher temperature.
In FIGS. 4A and 4B, the front end of the gate pulse input to the scanning line is slashed so that the gate voltage VG1 of the input pulse is approximately equal to the gate voltage V2 of the pulse transmitted to the end of the scanning line. Hence the coupled voltage VCOUPLED1 is approximately equal to VCOUPLED2 to avoid the flicker phenomenon.
Transistor is usually used as a switch as shown in the circuit in FIG. 2. Normally, TFT needs a longer period to be recharged when the temperatures is low dues the inferior mobility of the carriers. Nonetheless, the temperature characteristic of Transistor slashes the gate pulse more at lower temperatures. The slashes on the gate pulses reduce the recharging time of the TFT. Consequently, insufficient recharging time of TFT occurs at lower temperatures.